1. Field of the Invention
The present invention relates to a rapidly-solidified aluminum powder alloy having the so-called nanolevel fine structure with high strength and excellent toughness which is applicable to a part or a structural material requiring toughness, and relates to a method of preparing the same. More particularly, the invention relates to an aluminum alloy having a volume ratio of not more than 35 vol. % of an intermetallic compound that precipitated in a matrix, and to a method of preparing the same. The term "nanolevel structure" stands means a metallographic structure having a grain size not more than about several hundred nanometers (nm).
2. Description of the Background Art
Japanese Patent Laying-Open No. 64-47831 (1989) discloses an aluminum alloy having a nanolevel fine structure, which is obtained by heating rapidly-solidified aluminum alloy powder containing an amorphous phase and extruding the same.
Although the alloy obtained by the technique disclosed in this Laying-Open Publication No. 64-47831 has excellent strength (tensile strength and proof strength), its Charpy impact value is less than about 1/5 that of a conventional aluminum ingot material. Thus, it is difficult to employ this aluminum alloy as a material for a machine part or an automobile part which requires reliability.
On the other hand, the inventors have already proposed a method of employing rapidly-solidified aluminum alloy powder and heat treating its amorphous phase for powder-forging the same, in Japanese Patent Laying-Open No. 5-279767 (1993).
The technique proposed in the aforementioned Japanese Laying-Open No. 5-279767 is based on an idea of rapidly heating, then forging and thereafter rapidly cooling the powder for preventing the structure from developing coarseness and for attaining sufficient bonding strength between grains. However, this publication does not disclose any technique for forming a structure that is superior in strength and toughness by controlling the heating pattern in the heating step before forging.